The objective of this work is to characterize the histology and ultrastructure of tissue invasion and destruction in root caries lesions. The purpose is to establish structural standards in order to: (1) assess clinical variability of the disease, (2) judge the validity of animal models, and (3) to describe the nature of tissue invasion and destruction by suspected pathogens. Histological and ultrastructural examination of human root surface lesions, using standard techniques, will determine mechanisms of dentinal tubule penetration, matrix erosion, and pulpal pathology. The human root lesions will be compared and contrasted to lesions induced in experimental animal model systems by a cariogenic diet known to produce proliferation of indigenous saccharolytic microorganisms. The characterization of onset and progression of root caries in these animals may in the future, in the future lead to experimental manipulation directed toward prevention and treatment of the disease. Further, the nature of lesions developed in gnotobiotic rats by suspected pathogens will be compared to naturally occuring lesions in order to validate the histopathology of the experimental system. Similar comparisons to natural pathology will be made to lesions produced by these suspected pathogens and their products in vitro.